A high-output laser light source has been drawn attention as a light source to be used in a laser processing device, a laser display, or a like device.
A solid-state laser such as a YAG laser, a fiber laser using a fiber doped with a rare earth component such as Yb or Nd, or a like has been developed as a high-output laser light source for emitting laser beams in an infrared region. Meanwhile, a semiconductor laser using gallium arsenic, gallium nitride, or a like has also been developed as a high-output laser light source for emitting laser beams in red and blue regions. In a current technology, it is still difficult to emit green laser beams directly from a semiconductor, as high-output laser beams in a green region. In view of this, generally, high-output laser beams in a green region are emitted by subjecting laser beams in an infrared region, which are emitted from a solid-state laser such as a YAG laser, or a fiber laser, to wavelength conversion by a nonlinear optical element.
As examples of the nonlinear optical element, there have been developed elements (nonlinear optical elements) made of a nonlinear optical single crystal such as lithium niobate (LiNbO3), lithium tantalite (LiTaO3), lithium triborate (LiB3O5:LBO), β-barium borate (β-BaB2O4), titanyl potassium phosphate (KTiOPO4:KTP), or cesium lithium borate (C8LiB6O10:CLBO).
For instance, the following nonlinear optical element is used in a device for obtaining a laser output in a green region.
A quasi phase matching (QPM) wavelength conversion element made of a lithium niobate crystal having a polarization reversed structure is preferably used in a device capable of obtaining laser beams of 200 to 300 mW-class in a green region in the aspect of obtaining high conversion efficiency by a large nonlinear optical constant.
A nonlinear optical single crystal such as LBO or KTP is used in a device capable of obtaining high-output laser beams of several-watt-class in a green region.
The LBO crystal, however, has a small nonlinear optical constant. Accordingly, it is necessary to construct a resonator to obtain high conversion efficiency, and mount the LBO crystal in the resonator. This may make the configuration of the laser device complicated, and require fine adjustment on alignment.
As compared with the LBO crystal, the KTP crystal has a larger nonlinear optical constant. Accordingly, the KTP crystal is advantageous in obtaining high conversion efficiency without constructing a resonator. However, the KTP crystal has a disadvantage that the crystal is easy to be broken or degraded by a fundamental wave or a generated second harmonic wave.
In, addition, in lithium niobate or lithium tantalite, there are reported that a change in refractive index (photo refractive) by light, i.e. light damage, which is a phenomenon of crystal degradation, is suppressed by introducing an additive to the crystal, as recited in patent document 1, or by growing the crystal by a method capable of approximating the crystal composition close to an idealistic composition (stoichiometric composition).
In addition to the above, non-patent document 1 has reported an approach of generating 1.7 W green laser beams, as high-output laser beams in a green region, by heating an LiNbO3 crystal doped with 5 mol magnesium oxide to 140° C. Also, in recent years, non-patent document 2 has reported an approach of generating 3 W green laser beams by combining a wavelength conversion element made of a crystal substrate of an LiNbO3 single crystal doped with magnesium oxide subjected to periodical polarization reversal, with a fiber laser capable of narrowing the wavelength bandwidth of an oscillation wavelength.
In the following, an arrangement of a conventional wavelength conversion device incorporated with a nonlinear optical element is described referring to FIG. 14.
In the wavelength conversion device shown in FIG. 14, laser beams generated in a fundamental wave light source 101 propagate in the air, and are concentrated on a condenser lens 102, and incident into a wavelength conversion element 103. Then, a part of the fundamental wave incident into the wavelength conversion element 103 is subjected to wavelength conversion by the wavelength conversion element 103. A generated harmonic wave and the remaining fundamental wave are collimated into parallel beams by a re-collimator lens 104, and then separated into a harmonic 106 and a remaining fundamental wave 107 by a beam splitter 105. And, the remaining fundamental wave having a high-energy separated by the beam splitter 105 is wasted by a beam dumper 108.
As described above, the KTP crystal or the LBO crystal has the drawback that the crystal may be damaged or degraded by a second harmonic wave. In order to suppress the drawback, there is proposed an approach of suppressing crystal degradation, in which wavelength conversion is performed by using multiple wavelength conversion elements to lower the power density of a fundamental harmonic to be incident into each of the wavelength conversion elements (see e.g. patent document 3).
In the following, a wavelength conversion device incorporated with multiple wavelength conversion elements recited in patent document 3 is described referring to FIG. 15.
As shown in FIG. 15, a fundamental wave emitted from a fundamental wave light source 101 is concentrated by a condenser lens 102a, and then incident into a first wavelength conversion element 103a. After the wavelength of the fundamental wave is converted by the first wavelength conversion element 103a, the fundamental wave is collimated into a parallel beam by a collimator lens 104a. And, a harmonic wave 106a is separated by a beam splitter 105a. A remaining fundamental wave separated by the beam splitter 105a is concentrated by a condenser lens 102b, and then incident into a second wavelength conversion element 103b. After the wavelength conversion by the second wavelength conversion element 103b, the remaining fundamental wave is collimated into a parallel beam by a collimator lens 104b, and then separated into a harmonic wave 106b and a remaining fundamental wave 107 by a beam splitter 105b. Then, the remaining fundamental wave 107 is absorbed and diffused by a heat sink 108.
In the case where a 3 W harmonic wave is obtained by projecting a fundamental wave of e.g. 8 to 9 W to the conventional wavelength conversion device as shown in FIG. 15, a fundamental wave of 5 to 6 W is outputted as a remaining fundamental wave. The remaining fundamental wave is high-energy laser beam to be outputted as a parallel beam. In order to absorb and diffuse such a high-energy remaining fundamental wave, heat releasing means such as a large-sized beam dumper, a heat-releasing fin, or a heat sink has been required. Further, the above wavelength conversion device is a relatively large size because of requiring an arrangement of an optical component such as a lens or a beam splitter control at a predetermined position in a state such that beams run around in a free space.
Furthermore, although the aforementioned conventional wavelength conversion device may be usable in a large-sized apparatus such as a laser processing device, it is difficult to incorporate in a compact consumer product such as a laser display, which is proposed as a novel application of laser.
In a wavelength conversion device, a laser light source may be miniaturized by narrowing a wavelength band of a fundamental wave suitable for wavelength conversion, with use of a fiber laser. However, it is required to arrange a wavelength conversion element and various optical components in the similar manner as in the conventional arrangement. Accordingly, it is difficult to miniaturize the entirety of the wavelength conversion device even with use of a fiber laser.
Patent document 1: Japanese Patent No. 3,261,594
Patent document 2: Japanese Patent No. 3,424,125
Patent document 3: Japanese Unexamined Patent Publication No. Hei 11-271823
Non-patent document 1: Applied Physics letters, 59, 21, 2657-2659 (1991)
Non-patent document 2: Conference on Lasers and Electro-Optics 2005 (CLEO2005), Technical digest, CFL-1 (2005)